Q: When is a eucalypt not Eucalyptus? A: When it is Corymbia or Angophora
This post was inspired by an ID mistake I made recently. Hopefully I have now learned to not do the same in the future.
Most people are very familiar with eucalypt trees. In fact, most Australians would see one every day. They occur in almost all vegetation types across the continent in coastal, arid, tropical and alpine environments. We all have a fairly good idea of what a eucalypt looks like despite the vast array of growth forms, bark types and fruit shapes and sizes, but very few of us have a good handle on their names. If you asked the average person on the street to tell you the name of a eucalypt, many would start by suggesting it was called Eucalyptus something-or-other, and with over 700 species they will most likely be right.
However there are also a number of trees that are commonly refered to as eucalypts, and they look like Eucalyptus for the most part, but they are not. Technically there are 7 genera that occur within the tribe Eucalypteae (Wilson et al. 2005), however I am focusing on the two most commonly observed and misidentified genera: Corymbia and Angophora. These genera are both in the same family (i.e. Myrtaceae) and tribe (i.e. Eucalypteae) as Eucalyptus and are indeed very similar, but they have a few distinct spotting characters that can help you to tell them apart.
Angophora: This genus contains only 10 species and are often difficult to distinguish from Eucalyptus. Perhaps the most common distinguishing features are 1) the lack of an operculum (or cap) on the fruits and ribs along the fruits, and 2) the occurrence of opposite leaves on mature individuals. Since the operculum in Eucalyptus species is formed from the petals and or sepals, Angophora species are one of few eucalypts to have visible petals. Opposite leaves occur in many Eucalyptus species but typically only in the juvenile stages.
Corymbia: These are mostly ‘gums’ due to the bark type, and there are currently 113 species in the genus (Parra-O et al. 2009). Their primary distinguishing character is the inflorescence, which occurs as a ‘corymb’ i.e. a compound umbel, similar to the inflorescence commonly seen in the family Apiaceae. They possess an operculum on the fruits, as do Eucalyptus species. Some common species planted in suburban areas are the Red-flowering Gum, Lemon-scented Gum and the Spotted Gum.
Parra-O C. et al. (2009). Phylogeny, major clades and infrageneric classification of Corymbia (Myrtaceae), based on nuclear ribosomal DNA and morphology. Australian Systematic Botany, 22, 384–399
Wilson P. et al. (2005). Relationships within Myrtaceae sensu lato based on a matK phylogeny. Plant Systematics and Evolution, 251: 3–19